Tackling the Zero-Shot Reinforcement Learning Loss Directly

TL;DR

This paper demonstrates that the zero-shot reinforcement learning loss can be directly optimized for various priors, revealing insights into existing methods like VISR and highlighting limitations in feature diversity.

Contribution

It proves direct optimization of the zero-shot RL loss for multiple priors, connecting it to existing approaches and analyzing its implications.

Findings

White noise prior leads to an objective similar to VISR

Some priors produce narrow optimal features

Direct optimization offers new insights into zero-shot RL

Abstract

Zero-shot reinforcement learning (RL) methods aim at instantly producing a behavior for an RL task in a given environment, from a description of the reward function. These methods are usually tested by evaluating their average performance on a series of downstream tasks. Yet they cannot be trained directly for that objective, unless the distribution of downstream tasks is known. Existing approaches either use other learning criteria [BBQ+ 18, TRO23, TO21, HDB+ 19], or explicitly set a prior on downstream tasks, such as reward functions given by a random neural network [FPAL24]. Here we prove that the zero-shot RL loss can be optimized directly, for a range of non-informative priors such as white noise rewards, temporally smooth rewards, ``scattered'' sparse rewards, or a combination of those. Thus, it is possible to learn the optimal zero-shot features algorithmically, for a wide mixture of priors. Surprisingly, the white noise prior leads to an objective almost identical to the one in VISR [HDB+19], via a different approach. This shows that some seemingly arbitrary choices in VISR, such as Von Mises--Fisher distributions, do maximize downstream performance. This also suggests more efficient ways to tackle the VISR objective. Finally, we discuss some consequences and limitations of the zero-shot RL objective, such as its tendency to produce narrow optimal features if only using Gaussian dense reward priors.